Killer strains of yeast secrete a protein toxin to which they are themselves immune. Toxin and immunity are determined by the M dsRNAs - M1 (1.5 kb) for the K1 system and M2 (1.0 kb) for the K2 system. The replication of M1 or M2 depends on L-A dsRNA (4.5 kb), which encodes the major protein of the virus-like particles in which L-A and M dsRNAs are encapsulated. Natural variants of L-A carry combinations of the cytoplasmic genes [EXL], [HOK], and [NEX]. Most strains also carry another L-sized dsRNA (L-B or L-C) which is found in VLPs with a smaller coat protein. We found strains lacking all forms of L dsRNA and M dsRNA. In such strains we discovered two new species of dsRNA [T (2.7 kb) and W (2.25 kb)] present at low copy number. T and W are both cytoplasmically inherited and have no homology with other dsRNAs, cellular DNAs, or each other. The amount of T and W in the cell is induced about 10-fold by growth of cells at 37 C. Their replication is independent of L or M dsRNAs. T and W are not found in the same VLPs as are L and M dsRNAs. Mutants in ski genes suppress the exclusion of M2 dsRNA by L-A-HN that is seen in an mktl host. Using this, we have discovered three new ski genes and two new other genes (mks8 and MKS50) suppressing this effect of L-A-HN. The ski- mutations also increase M copy number, make the cells superkillers (increased toxin production), make the host cold-sensitive for growth, and enable M1 to use L-A-E instead of L-A-HN to support its replication. The host cold sensitivity is eliminated if M dsRNA is eliminated. The LTS5 gene is essential for both host growth at low temperature and for M dsRNA replication. We propose that high M copy number, due to the ski-mutation, diverts the LTS5 product needed for host growth at low temperature. Elimination of M1 or M2 dsRNAs results in increased L-A and total L-A + L-(BC) copy number in either SKI+ or ski- hosts. The ski2-2 mutation increases copy number of L-(BC) and of total L dsRNA. This suggests that M and L compete for some component necessary for replication. We find that M1 and M2 dsRNAs show frequent size variation of as much as 500 nucleotides within 100 generations. M2 and L-A-E, but not L-A-HN, need polyamines. M1 and M2 need sperimine or spermidine, while putrescine is sufficient for L-A-E.